Natural gas hydrate, commonly known as “combustible ice”, is widely distributed in the continental permafrost regions, the slopes of islands, the uplifted zones of the active and passive continental margins and some deep-water environments of the inland lakes in nature. In view of the particularity of the forming and reserving conditions of natural gas hydrates, so far the distribution of the vast majority of the gas hydrates in the world has been indirectly confirmed by the evidence obtained from geophysical surveys, except for a few samples of natural gas hydrates obtained from subsea drilling and seabed sediment sampling. Although significant progress has been made in the understanding of the reserves, distribution and characteristics of natural gas hydrates, people still need to conduct more scientific drilling and wellbore testing to further improve the understanding of the hydrates in terms of energy resources, geological hazards and impacts on the climate changes; however, a series of key scientific problems and technical challenges need to be settled before this.
The formation and stabilization of natural gas hydrates require an exceptional high-pressure and low-temperature environment. During the drilling process of natural gas hydrates, a large amount of heat energy will be produced by the process of rock cutting with a drill bit and the friction between the drilling tool at the bottom of the well and the well wall and rock core, as well as the release of the formation stress near the wall and the bottom of the well, which will result in the decomposition of natural gas hydrates into gas and decomposed water, thus greatly reducing the geomechanical stability of the sediments. Because the mineral reserves of natural gas hydrates, especially those in sea areas, are usually present at a relatively shallow depth, risks such as gas seepage, gas leakage and sleeve collapse are more likely to occur during the drilling process of natural gas hydrates compared with conventional oil and gas reserves.
In view of the foreseeable risks during the drilling process of natural gas hydrates, it is necessary to study the producing mechanism and the controlling method of all the geological hazards related to the drilling process of natural gas hydrates with various characteristics. A limited number of trial exploitations of the hydrates carried out internationally are all focused on the studies on the geological changes and hazards that may be caused during the drilling process. For example, the Natural Gas Hydrate Combined Industry Program in the Gulf of Mexico, established in 2001, is part of the effort on studies of the hazards associated with well drilling in the hydrate sediments. However, the field testing needs a large sum of money and is extremely costly, and it is only suitable for countries where the physical samples of the gas hydrates have been identified. Therefore, it is an effective way and a necessary means to reduce the risks in the drilling exploitation by simulating the forming environment of natural gas hydrate reserves and studying the law and the influence mechanism of the drilling process through the establishment of experimental simulation instruments and equipment in the laboratory. Because the experimental simulation studies are cost-saving and lay a foundation for other studies, the experimental-simulation studies on natural gas hydrate drilling have become the most feasible method for studying natural gas hydrate drilling technology.
In the process of laboratory simulations, the drilling fluid, which is used for conveying the solid particulates produced dining the drilling process from the downhole location where it is drilled to the outside, is typically pumped down to the drill bit along a hollow drill stem when a wellbore is drilled into a simulated formation. Different from conventional drilling, the drilling fluid mixture returned from the drilling process of natural gas hydrates carries a large amount of gas and undecomposed hydrate. In order to accurately understand the downhole drilling process, it is necessary to measure the drilling fluids gas carrying and water producing capacities in real time. Because there is a need to maintain a high-pressure and low-temperature state in the simulation process of the hydrate formation, it is necessary to separate the drilling fluid mixture under a high pressure for the polyphase separation so as to facilitate the measurement. Meanwhile, it is also necessary to prevent the formation of new hydrates resulting from the reaction of the gas and the liquid during the process of the separation so as to avoid equipment blockages and inaccurate measurements.
Therefore, there is a need to provide a device for separating the high-pressure and high-speed natural gas hydrate drilling fluid to ensure the separation of the small dry abrasive particles produced during the drilling process from the drilling fluid so as to conduct real-time separated measurement of the gas and liquid carried thereof.